The present invention relates to a GPS (Global Positioning System) receiving apparatus. More particularly, the invention relates to an apparatus and a method for synchronously acquiring and holding the spread code and carrier wave regarding signals received from GPS satellites.
In a GPS setup for measuring the position of a moving object by use of artificial satellites (GPS satellites), the basic function of a GPS receiving apparatus on board the object involves receiving signals from at least four GPS satellites, computing the position of the apparatus based on the received signals, and informing a user of the computed position.
In operation, the GPS receiving apparatus first decodes signals received from the GPS satellites (the signals are called the GPS satellite signals hereunder) to acquire orbit data about each GPS satellite. The GPS receiving apparatus then acquires its own three-dimensional position based on orbit and time information from the GPS satellites as well as on delay times of the GPS satellite signals using simultaneous equations. The signals from as many as four GPS satellites are needed for positioning computation because errors detected as differences between the internal time of the GPS receiving apparatus and the time of each satellite must be inhibited from aversely affecting the computation.
Civilian-use GPS receiving apparatuses perform their positioning computation by receiving spread spectrum signal waves called C/A (Clear and Acquisition) code on an L1 band from GPS satellites (of NAVSTAR).
The C/A code is a signal obtained by modulating a 1575.42-MHz carrier wave (called carrier hereunder) in BPSK (Binary Phase Shift Keying) using a PN (pseudorandom noise) series code such as Gold code having a chip rate of 1.023 MHz and a code length of 1,023 chips with 50-bps data spread therein. With its code length of 1,023 chips, the C/A code is a PN series code repeated at a rate of 1,023 chips per cycle (thus one cycle=1 millisecond), as shown in FIG. 41A.
The PN series code (spread code) used as the C/A code varies from one GPS satellite to another. The GPS receiving apparatus detects beforehand which GPS satellite uses what kind of PN series code. Navigation messages, to be discussed later, allow the GPS receiving apparatus to determine from which GPS satellites signals can be received in the present apparatus position at the present point in time. This makes it possible for the GPS receiving apparatus in a 3D positioning process to receive radio waves from at least four GPS satellites that can be acquired from where the apparatus is currently located. Upon receipt of the radio waves from the satellites, the GPS receiving apparatus performs spread spectrum decoding and positioning computation to find the current position of the apparatus.
As shown in FIG. 41B, 1 bit of satellite signal data (navigation message data) is transmitted in 20 periods of the PN series code. In other words, the satellite signal data are transmitted in increments of 20 milliseconds or at a rate of 50 bps. One period of the PN series code corresponds to 1,023 chips that are inverted depending on the bit being “1” or “0.”
In the GPS setup, as shown in FIG. 41C, one word is made up of 30 bits (in 600 milliseconds). Ten words constitute one sub-frame (6 seconds) as depicted in FIG. 41D. As illustrated in FIG. 41E, the first word of one sub-frame has a preamble inserted therein which maintains a predetermined bit pattern regardless of the data being updated. The preamble is followed by transmitted data.
Five sub-frames make up one frame (30 seconds). Navigation message data are transmitted in increments of frames. The first three sub-frames in one frame constitute satellite-specific orbit information called ephemeris information. Ephemeris information is transmitted repeatedly, i.e., in increments of main frames (at intervals of 30 seconds). This information includes parameters by which to obtain the orbit of the satellite sending the information, and a signal transmission timestamp from the satellite.
The second word in the three sub-frames that make up the ephemeris information includes time data called a week number and TOW (time of week). The week number is information that is incremented weekly starting from Jan. 6 (Sunday), 1980 taken as week zero. TOW is time information that is incremented in units of six seconds starting from 00:00 on Sunday (i.e., incremented at intervals of sub-frames).
All GPS satellites have an atomic clock each and operate on common time information. Signal transmission timestamps from the GPS satellites are synchronized with their atomic clocks. Based on the two pieces of time information received (week number and TOW), the GPS receiving apparatus obtains an absolute time. In synchronizing with and locking onto radio wave transmissions from a satellite, the GPS receiving apparatus synchronizes its own time data shorter than six seconds with the time of that satellite within an accuracy specific to a reference oscillator incorporated in the GPS receiving apparatus.
The PN series code of each GPS satellite is generated in synchronism with its atomic clock. The ephemeris information from a given GPS satellite allows the GPS receiving apparatus to acquire the position and speed of that satellite which are used in positioning computation.
Ephemeris information constitutes a highly accurate calendar that is updated with relatively high frequency under control of ground-based control stations. The GPS receiving apparatus keeps the ephemeris information in a memory for use in positioning computation. Because of its inherent accuracy level, the ephemeris information obtained at a given point in time generally has a useful life of about two hours. Under the life time constraint, the GPS receiving apparatus monitors the elapse of time starting from the time that the ephemeris information is placed into the memory. When the useful life is judged to have elapsed, the GPS receiving apparatus updates the ephemeris information in the memory.
It takes at least 18 seconds (3 sub-frames) for the GPS receiving apparatus to acquire new ephemeris information and substitute the obtained information for what is currently held in the memory. If data are obtained halfway through a sub-frame period, a total of 30 seconds is required for the update.
The navigation message constituted by the remaining two sub-frames in the one-frame data is called almanac information that is transmitted commonly from all GPS satellites. A total of 25 frames of almanac information is needed to acquire all necessary information. Almanac information includes an approximate position and availability of each of the GPS satellites.
Almanac information is updated at intervals of several days under control of ground-based control stations. The GPS receiving apparatus can use almanac information for its operations by retaining it in the memory. The stored almanac information is considered to have a useful life of several months. Generally, the GPS receiving apparatus acquires new almanac information from a GPS satellite at intervals of a few months and substitutes the newly obtained information for what is retained in the memory. The almanac information kept in the memory allows the GPS receiving apparatus to compute channels to which to allocate satellites upon power-up.
When the GPS receiving apparatus is to obtain the above-described data upon receipt of GPS satellite signals, it is necessary for the apparatus to prepare in advance and utilize the same PN series code as the C/A code employed by each GPS satellite whose signal is to be received (the PN series code is called the PN code hereunder). In operation, the GPS receiving apparatus performs phase synchronization with the C/A code to acquire the signal from a given GPS satellite and subject what is acquired to spread spectrum decoding. After successful phase synchronization with the C/A code and following spread spectrum decoding, the GPS receiving apparatus can detect bits and a navigation message including time information from the GPS satellite signal.
A GPS satellite signal is acquired by making a search for phase synchronization with the C/A code. In the phase synchronization search, the GPS receiving apparatus detects a correlation between its own PN code and that of the signal received from the GPS satellite in question. If the detected correlation value is judged to be greater than a predetermined value, the two PN codes are considered to be in synchronism. If phase synchronization is not judged obtained, a suitable synchronizing technique is used to control the phase of the PN code on the side of the GPS receiving apparatus so as to achieve synchronism with the PN code of the received signal.
As discussed above, each GPS satellite signal is a signal furnished by modulating a carrier through BPSK with a signal involving spread code data. For this reason, the GPS receiving apparatus attempting to receive the signal from a given GPS satellite needs to gain synchronism not only with the spread code but also with the carrier and data. However, it is impossible to achieve synchronization independently with the spread code and with the carrier.
Generally, the GPS receiving apparatus converts the carrier frequency of a received satellite signal into an intermediate frequency of several MHz or less in order to use the intermediate frequency signal in the synchronous detection process above. The carrier of the intermediate frequency signal contains two kinds of error: a frequency error due to a Doppler shift proportionate to the moving speed of the GPS satellite in question, and a frequency error attributable to local oscillator deviations inside the GPS receiving apparatus.
These frequency errors render the carrier frequency of the intermediate frequency signal unpredictable. Hence the need for a frequency search. A synchronization point (synchronization phase) in one spread code period is contingent on the positional relation between the GPS receiving apparatus and the GPS satellite of interest and is thus unpredictable. This requires putting some suitable synchronizing technique in place, as mentioned above.
Conventional GPS receiving apparatuses detect synchronization with the carrier and spread code using sliding correlation involving a frequency search. At the same time, the receiving apparatuses to date use DLL (delay locked loop) and Costas loop arrangements for synchronous acquisition and synchronous hold. These capabilities will be explained further below.
A clock signal for driving a PN code generator in the GPS receiving apparatus is generally obtained by dividing the output of a reference frequency oscillator incorporated in the apparatus. The reference frequency oscillator is typically constituted by a highly accurate crystal oscillator. Out of the output from the reference frequency oscillator, a locally oscillated signal is generated for use in converting the received signal from a given GPS satellite into an intermediate frequency signal.
FIG. 42 is an explanatory view of the above-mentioned frequency search. If the clock signal for driving the PN code generator in the GPS receiving apparatus has a frequency f1, a phase locked search for the PN code is carried out. That is, while the PN code phase is being shifted one chip at a time, a correlation is detected between the received GPS signal in each chip phase and the PN code. This permits detection of peak correlation values, whereby the phase enabling PN code synchronization (spread code phase) is detected.
If no synchronous phase is detected for each of 1,023 chips where the clock signal has the frequency f1, that means carrier frequency synchronization is not obtained. In such a case, the dividing ratio of the reference frequency oscillator is illustratively varied so as to switch the driving clock signal to a different frequency f2. A phase search is then carried out similarly for each of 1,023 chips. The frequency of the driving clock signal is switched in stepped fashion as indicated in FIG. 42 for repeated searches. This is how the frequency search is implemented.
If the frequency of a synchronous driving clock signal is detected during the frequency search, the detected clock frequency is used for a final synchronous acquisition of the PN code. A carrier frequency is detected from the clock frequency. This allows the GPS receiving apparatus to acquire a given satellite signal despite deviations of the oscillation frequency from the internal crystal frequency oscillator.
There is a drawback to the above conventional method for performing synchronous acquisition and synchronous hold simultaneously regarding the carrier and spread code using sliding correlation with a frequency search and DLL (delay locked loop) and Costas loop arrangements. That is, the technique of sliding correlation with a frequency search is theoretically unfit for high-speed synchronization as discussed above. Thus it takes time to achieve spread code and carrier synchronization. The prolonged process of carrier and spread code synchronization results in a slower response of the GPS receiving apparatus inconveniencing the user in practice.
In order to overcome the drawback above, actual GPS receiving apparatuses to date are required to adopt a multi-channel approach in implementing parallel searches for synchronization points. Allocating a large number of channels based on the conventional method can make the GPS receiving apparatus more complex structurally and more costly. The multi-channel scheme with parallel searches for synchronization points dissipates more power. A power-hungry feature can be a serious disadvantage especially in portable GPS receivers.
With the conventional method in place, synchronous acquisition and synchronous hold of the spread code and carrier are implemented integrally with sliding correlation involving a frequency search and with DLL (delay locked loop) and Costas loop arrangements. It follows that if the signal from a given GPS satellite is lost, synchronous acquisition and synchronous hold must be again carried out integrally, which can be a time-consuming process.
Furthermore, because the conventional method involves carrying out synchronous acquisition and synchronous hold of the spread code and carrier in integral relation with sliding correlation entailing a frequency search and with DLL (delay locked loop) and Costas loop arrangements, any attempt to raise the sensitivity of the GPS receiving apparatus would in theory prolong the processing time appreciably for synchronous acquisition and synchronous hold. Bypassing this bottleneck has turned out to be more difficult than expected.
It is therefore an object of the present invention to provide a GPS receiving apparatus and a GPS satellite signal receiving method for raising the speed of synchronous acquisition and synchronous hold of GPS satellite signals while boosting the sensitivity of the apparatus with ease.